Method and apparatus for emphasizing sharpness of image by detecting the edge portions of the image

ABSTRACT

According to this invention, a sharpness emphasis is carried out by multiplying a sharpness emphasis signal of an original image by a function of a look-up table which is accessed based on an output from an edge detecting filter performing detections in accordance with only the edge portions of the original image. Since the edge detecting filter for detecting edges of the image is equipped in order to emphasize the sharpness only in the edge portions of the image, it is possible to prevent the image grains from being sharpness-emphasized. In addition, although the edge detecting filter may have a same size of a mask with that of the unsharpness mask, it is possible, if required, to modify freely both sizes of the masks, in order to perform a proper process in accordance with each image variation.

This is a Continuation of application Ser. No. 07/674,985, filed Mar.26, 1991, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for emphasizingsharpness of an image to be used for a color scanner for mask-making, alaser color printer, a facsimile or the like.

In the prior art image scanning recording apparatus, such as a colorscanner, there has been employed an electro-optical unsharpness(differential) masking method, an electrical crisping or a peakingmethod in order to improve the sharpness of an image. These methods areeffective to improve the sharpness and reproduce a recorded imagesharply, however, at the same time, since the improvement of thesharpness is effected uniformly over the whole area of an originalimage, the grain clusters of the original film are disadvantageouslyemphasized also in an area having a flat tone on the original. As aresult, a poor quality of the recording image having uneven, roughgraininess occurs.

In order to solve the problem described above, there has been introduceda method for emphasizing sharpness disclosed in Japanese PatentApplication Laid-open No.146451/1980. In this method, a sharpness signalis formed based on a main signal obtained from a photoelectricallyscanned original, or based on the main signal, and an unsharp signalwhich includes the information on the peripheral area of the scannedarea corresponding to the main signal. This sharpness emphasis signalcorrects or modifies the main signal so as to emphasize the image to berecorded. When this process is carried out, the variation level of theunsharp signal is measured or distinguished so that the quantity of thesharpness emphasis signal can be controlled on the basis of the measuredvariation level. In this method, no reference was made to the method ofthe auxiliary scanning, however, and the sharpness emphasis is carriedout in accordance with only the main scanning. The lack of auxiliaryscanning disadvantageously causes the deterioration of the imagequality. In addition, since the unsharp signal used for the sharpnessemphasis and another unsharp signal to distinguish an edge arerespectively formed of the same masks having a fixed size, the abovemethod exhibits a weakness in dealing with the image variationrequirements, such as a screen line number, edge width or the like.

On the other hand, other methods for emphasizing sharpness do not havesuch defects as described above, such methods are disclosed by thepresent applicant in Japanese Patent Application Laid-open No.3269/1985and Japanese Patent Application Laid-open No.54570/1985. However, sinceit is impossible for the methods to distinguish between the grain(coupler) and the edge (film; especially a reversal film), the sharpnessin the flat area of the image is likely to be emphasized, whichconsequently leads to an unevenness of the image quality.

SUMMARY OF THE INVENTION

This invention has been achieved in consideration of the above discussedcircumstances and an object of the present invention is to provide amethod and its apparatus by which a sharpness emphasis can besufficiently effected in proper manner without emphasizing thegraininess of grains on the original image.

According to one aspect of this invention, for achieving the objectsdescribed above, there is provided an image emphasis method which ischaracterized in that; when a sharpness emphasis signal of an image ismultiplied by sharpness coefficients, the sharpness of the image isemphasized by multiplying the sharpness coefficients by a function valuewhich is outputted from a look-up table based on the output of an edgedetecting filter for detecting only edge portions of the image.

According to another aspect of this invention, there is provided animage sharpness emphasizing apparatus comprising; a line memory whichreceives a sharp signal of an image to output an unsharp signal and amatrix signal, an edge detecting filter for detecting edges on theimage, a multiplication accumulating means which multiplies the matrixsignal by the output of the edge detecting filter so as to beaccumulated, a subtracting means which calculates a difference signalbetween the sharp signal and the unsharp signal, a look-up table to beaccessed based on the output value of the multiplication accumulatingmeans, a multiplication means for effecting multiplications between thesaid difference signal, the sharpness coefficients and a function outputof the above-mentioned look-up table, and an adder for adding up thesharp signal and the output of the multiplication means so as to beoutputted.

The nature, principle and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a flow chart showing an example of the method of thisinvention;

FIGS. 2A and 2B are examples of the edge detecting filters used for thisinvention;

FIG. 3 is a plotted diagram showing an example of a look-up table (edgedependence table) for use in this invention;

FIG. 4 is a diagram for explaining the operation of this invention;

FIG. 5 is a block diagram showing an operational example of theapparatus according to this invention;

FIG. 6 is a block diagaram for partly illustrating another embodiment ofthis invention; and

FIGS. 7A and 7B are diagrams showing respective edge detecting filtersfor other examples.

DESCRIPTION OF THE PREFERRED EMBODIMENT

According to this invention, a sharpness emphasis is carried out bymultiplying a sharpness emphasis signal of an original image by afunction value of a look-up table (edge adjusting table) which isaccessed based on the output from an edge detecting filter performingdetections in accordance with only the edge portions of the originalimage.

FIG. 1 shows a flow chart showing a method of the sharpness emphasisaccording to the present invention. With reference to this flow chart,at first, an original image is read to generate a sharp signal S_(ij)(Step S1). Image data extracted is having a mask size of n×n in whichthe sharp signal S_(ij) exists as central data (Step S2). This processis followed by the Step S3 generating an edge detection level |E| aswell as by the Step S5 generating an unsharp signal u. In the Step S3,the edge detection level |E| is calculated according to formula (1)below ##EQU1## In this expression, the constant "k" is k=(n-1)/2 and"Mxy" indicates an edge detecting filter. Here, when the mask size ofthe unsharp signal u is taken, for example, as 5×5, the matrix for theedge detecting filter Mxy can be considered to be taken as the one shownin FIG. 2A or 2B. It may be possible to make a matrix more weighted. Itis also possible, for example to adopt the matrix of FIGS. 2A and 2B asfor the matrix of the main scanning direction and for the auxiliarydirection, respectively. After the process Step S3 in which the edgedetection level |E| is determined, an edge adjusting quantity f(|E|) isdetermined by using an edge adjusting table f() as shown in FIG. 3 (StepS4). The edge detection level (|E|) is determined by using an edgeadjusting table f() as shown in FIG. 3 (Step S4).

On the other hand, the unsharp signal u is calculated according to thefollowing formula (2) in the Step S5: ##EQU2## Subsequently, thedifference (S_(ij) -u) between the sharp signal S_(ij) and the unsharpsignal u is calculated to produce a sharpness emphasis signal (Step S6).Then, the multiplication of the sharpness emphasis signal (S_(ij) -u) bythe edge adjusting quantity f(|E|) is carried out following theexpression (3) shown below (Step S7).

    f(|E|)·(S.sub.ij -u)            (3)

After this process, the sharp signal S_(ij) is added to the above resultso as to output the sharp signal S_(ij) ' being sharpness-emphasized(Step S8). This is shown as the expression (4) below;

    S.sub.ij '=S.sub.ij +f(|E|)·(S.sub.ij -u)(4)

According to the method for sharpness emphasis of this invention asdetailed above, if a function f(|E|) does not affect any sharpnessemphasis as long as the edge detection level |E| is a small value asstored in the edge adjusting (FIG. 3), the sharpness emphasis is carriedout as shown in (C) of FIG. 4 in relation to the sharp signal shown as(A) in FIG. 4. The emphasis is done only when the edge detection level|E| exceeds the threshold TH, as shown in (B) of FIG. 4. That is, onlywhen the edge is surely detected.

FIG. 5 is a block diagram showing an example of the apparatus for thesharpness emphasis according to this invention. A sharpness signal S isstored in a line memory unit 1 as well as input to a subtracter 7 and anadder 6. After the input of the sharp signal S, the line memory unit 1outputs an unsharp signal u, and at the same time, outputs a sharpsignal I in the form of the n×n matrix (Step S2). The output M of anedge detecting filter 3 and the sharp signal I, both of which have aform of the matrix like the ones shown in FIGS. 2A and 2B, are inputinto to a multiplication accumulator 2 to be subjected to themultiplication accumulating operation as represented by the formula (1),in order to determine an edge detection level E (Step S3). A look-uptable 4 which holds a function as shown in FIG. 3, outputs a function inaccordance with the edge detection level E.

While, the unsharp signal u outputted from the line memory unit 1 isinput into the subtracter 7, in which the difference (S-u) between thesharp signal S and the unsharp signal u is calculated (Step S6). Theobtained result is input into a multiplier 5. The multiplier 5 to whichis inputted a sharpness emphasis coefficient K as well as the functionaloutput f(E) of the look-up table 4, performs an operation as shownbelow;

    K·(S-u)·f(E)                             (5)

Then, the resultant of the above expression (5) is inputted to the adder6 and added to the sharp signal S to output a sharpness-emphasized sharpsignal S (Step S8).

    S'=S+K·(S-u)·f(E)                        (6)

In FIG. 6, which shows another example of this invention, a plurality ofthe edge detecting filters (shown in FIGS. 2A and 2B) are introduced,respectively, for the main scanning direction and the auxiliary scanningdirection. A sharp signal X_(ij) of matrix outputted from the linememory 10 is inputted to a multiplication accumulator 11 for mainscanning and to a multiplication accumulator 12 for auxiliary scanning.Then the output F_(ij) of the edge detecting filter (FIG. 2A) and theoutput E_(ij) of the edge detecting filter (FIG. 2B) are respectivelyinput into the multiplication accumuator 11 for the main scanning andthe multiplication accumulator 12 for the auxiliary scanning, to outputthe edge detection level F for the main scanning direction and the edgedetection level E for the auxiliary sacanning direction to bemultiplication-accumulated. The edge detection levels F and E are inputinto respective absolute value forming circuits 13 and 14 in which thevalues are transformed to respective absolute values. The resultantabsolute value siganals |F| and |E| are added in an adder 15 to be inputinto a look-up table 16. The look-up table 16 holds a function as shownin FIG. 3 to output f(|E|)+|F|) in accordance with "|F|+|E|".

Accordingly, since this apparatus detects an edge in accordance withplural directions, i.e., by the main scanning direction and theauxiliary scanning direction, it is possible to perform edge detectionmore precisely. It should be noted that although the absolute valueforming circuit is employed in this configuration, a cicuit in which E²,F² are calculated can also be used in the present invention.

Other examples of the edge detecting filters are shown in FIGS. 7A and7B, which can be used as an edge detecting filter without weight,respectively and another one with weight. Also these edge detectingfilters can achieve the edge detections as described above.

According to the sharpness emphasis method and its apparatus of thepresent invention, since the sharpness emphasis is to be carried outwhen the edge detection level has a high value, it is possible to avoidemphasizing the grains and the like on the original. In addition, sincethe size of the mask is variable, and edges are detected from the pluraldirections, it is possible to effect an optimum sharpness emphasissteadly in accordance only with the edges.

It should be understood that many modifications and adaptations of theinvention will become apparent to those skilled in the art and it isintended to encompass such obvious modifications and changes in thescope of the claims appended hereto.

What is claimed is:
 1. A method of emphasizing sharpness in a picturesignal, said method comprising the steps of:(a) generating a picturesignal which represents an original image; (b) storing said picturesignal, and generating a matrix signal, having said picture signal ascentral data, and an unsharp signal, having peripheral data of saidpicture signal; (c) detecting edge levels of said matrix signal; (d)adjusting detected edges and generating an edge adjusting signal from afunction stored in a look-up table, said edge adjusting signal beinggenerated in accordance with said detected edge level; and (e)emphasizing sharpness in said picture signal in accordance with saidedge adjusted output signal and said unsharp signal.
 2. The method ofemphasizing sharpness recited in claim 1, wherein said detecting step(c) further comprises detecting said edges of said matrix signal for aplurality of different scanning directions and generating a plurality ofedge level signals of said matrix signal.
 3. The method of emphasizingsharpness recited in claim 2, wherein said detecting step is performedin a plurality of different scanning directions.
 4. The method ofemphasizing sharpness recited in claim 1, wherein said emphasizing stepfurther comprises:producing a difference signal representative of adifference between said picture signal and said unsharp signal;producing a product signal representative of a product of saiddifference signal and said edge adjusted output signal; producing anaddition signal representative of an addition of said picture signal andsaid product signal.